1. Field of the Invention
The present invention relates to a method and apparatus for controlling refrigeration cycle, by which revolution speed of a compressor can be controlled by an electronically commutated, brushless DC motor composed of a PWM-controlled inverter and a permanent magnet AC motor during refrigeration cycle to change the refrigeration capacity thereof.
2. Description of the Prior Art
In general, the speed of a compressor used in refrigeration cycle of an air conditioning apparatus or a refrigerating apparatus is controlled in accordance with pulse width modulation (PWM) control by use of an inverter.
In the inverter, a converter for converting AC power to DC power is provided on an input side of the inverter. The inverter and the converter are both composed of semiconductor switching elements. However, since the semiconductor switching elements are easily damaged by an over-current, the following protective method, for instance has been so far adopted: current flowing through a DC circuit is always detected; and when the detected current value increases to a limit value previously determined as a protective operating point, the output of the inverter is immediately interrupted to stop the compressor motor. In this control method, the operation delay time is lengthened with decreasing rise rate (di/dt) of the current, and shortened with increasing rise rate thereof.
In the above-mentioned conventional control method, however, the protective operation starts in response to a transient over-current to which the protective operation is not necessarily required, so that the compressor is often stopped. Once the compressor is stopped, it takes a relatively long time to restart the compressor.
In addition, when an over-current is generated at a high rise rate, the protective operation is often delayed, in spite of the fact that the protective operation is immediately required, thus resulting in a problem in that the semiconductor switching elements are often broken down.